International Patent Application WO 92/10270 discloses a process for the purification of a gas containing hydrogen sulphide, in which the gas is scrubbed in a gas scrubber with an alkaline scrubbing liquid, the scrubbing liquid used is treated in an aerobic reactor with oxygen in the presence of sulphur-oxidising bacteria the effluent from the aerobic reactor is used again as scrubbing liquid and the elemental sulphur formed during the treatment with oxygen is separated from the effluent. The process is suitable for the removal of hydrogen sulphide (H.sub.2 S) and other reduced sulphur compounds, such as mercaptans and carbon disulphide, or for the removal of sulphur dioxide (SO.sub.2). For H.sub.2 S removal, the pH is kept between 8 and 9 by means of buffering compounds. The sulphur-oxidising bacteria used include the genera Thiobacillus and Thiomicrospira.
A disadvantage of this known process is that relatively large quantities of scrubbing liquid are necessary in order to absorb the hydrogen sulphide effectively. The known process is also unsuitable for the removal of other contaminants which may be present in addition to H.sub.2 S, such as carbonyl sulphide (COS).